Assays for determining a variety of body-fluid analytes in a doctor's office or home setting are available. As a rule, such assays are designed for simplicity of use, on the assumption that the user has little or no training in clinical assay procedures or in reading and interpreting the results of the assay. For this reason, the assays tend to favor one-step analyte addition, with assay results being determined after a reaction end-point is reached. Analyte-addition assays of this type, while convenient for the user, generally lack accuracy and reliability. In particular, the test result may be subject to variations in reaction variables such as reagent levels and activities, particularly where enzyme reagents are involved, and temperature conditions. The accuracy and reliability of many simple assay procedures is also limited by background interference which can vary among analyte samples, and which may also be affected by the condition of the test reagents.
One blood analyte which is frequently assayed in a doctor's office setting, often as part of a cardiovascular examination, is serum cholesterol. Cholesterol is present in the blood either as free cholesterol (about 25%) or in esterified form (about 75%). Total cholesterol levels in normal individual is about 175 mg/dl, although levels as high as 600-700 mg/dl are present in serious hypercholesteremic conditions. It is well established that high blood cholesterol level, and particularly a high level of esterified cholesterol associated with low density lipoproteins (LDL), is directly associated with a number of serious disease conditions in humans, including coronary artery diseases, biliary obstruction, and liver or thyroid dysfunctions.
Cholesterol assays for use in a doctor's office setting are commercially available. However, despite the importance of cholesterol level as an indicator of a number of important disease states, these assays have proven to have limited accuracy and/or reliability, for the reasons discussed above. For example, the patient serum may inhibit color development on the pad, with the result that the cholesterol measurements may be spuriously low. Alternatively, enzyme reagents in the pad may have aged or otherwise denatured, also leading to low readings.